1. Field of the Invention
The present invention relates to wireless communications and, more particularly, to a method and apparatus for transmitting a feedback in a wireless communication system.
2. Related Art
The institute of electrical and electronics engineers (IEEE) 802.16e standard was adopted in 2007 as a sixth standard for international mobile telecommunication (IMT)-2000 in the name of ‘WMAN-OFDMA TDD’ by the ITU-radio communication sector (ITU-R) which is one of sectors of the international telecommunication union (ITU). An IMT-advanced system has been prepared by the ITU-R as a next generation (i.e., 4th generation) mobile communication standard following the IMT-2000. It was determined by the IEEE 802.16 working group (WG) to conduct the 802.16m project for the purpose of creating an amendment standard of the existing IEEE 802.16e as a standard for the IMT-advanced system. As can be seen in the purpose above, the 802.16m standard has two aspects, that is, continuity from the past (i.e., the amendment of the existing 802.16e standard) and continuity to the future (i.e., the standard for the next generation IMT-advanced system). Therefore, the 802.16m standard needs to satisfy all requirements for the IMT-advanced system while maintaining compatibility with a mobile WiMAX system conforming to the 802.16e standard.
Technology for supporting reliable and high-speed data service may include multiple input multiple output (MIMO). MIMO technology can be used to improve the efficiency of data transmission and reception using multiple transmission antennas and multiple reception antennas. MIMO technology includes spatial multiplexing, transmit diversity, beam-forming and the like. An MIMO channel matrix according to the number of reception antennas and the number of transmission antennas can be decomposed into a number of independent channels. Each of the independent channels is called a layer or stream. The number of layers is called a rank.
A user equipment (UE) may transmit a feedback through uplink. The feedback may include channel information necessary for data transmission. A base station (BS) may schedule radio resources using the feedback received from the UE and transmit data. A closed loop scheme is a scheme of transmitting data by compensating for channel information included in a feedback from UE, and an open loop scheme is a scheme of transmitting data by not compensating for channel information included in a feedback from UE. A feedback may not be transmitted according to the open loop scheme, and a BS may do not use channel information included in a feedback although the channel information is included in the feedback. In general, in a wireless communication system, the open loop scheme may be applied to a channel environment for UE that moves at high speed, and the closed loop scheme may be applied to a channel environment for UE that moves at low speed. The open loop scheme is applied to a channel for UE that moves at high speed because the channel is greatly changed, making channel information, included in a feedback, difficult to be reliable. The closed loop scheme may be applied to a channel environment for UE that moves at low speed because the channel environment is relatively less changed and channel information included in a feedback is reliable and less sensitive to delay.
An uplink control channel may carry various kinds of pieces of feedback information. The pieces of feedback information may include a channel quality indicator (CQI), an MIMO feedback, acknowledgement/non-acknowledgement (ACK/NACK), an uplink synchronization signal, a bandwidth request, etc. The pieces of feedback information may be combined in various ways according to a channel environment or a predetermined configuration and transmitted. Furthermore, feedback information about the entire bandwidth may be measured and fed back and feedback information about some subbands of the bandwidth may be measured and fed back. If feedback information about some subbands is measured, the indices of the relevant subbands may also be fed back.
There has not yet been proposed a detailed method of feeding a subband index back in an IEEE 802.16m system. Accordingly, it is necessary to propose a method of efficiently feeding a subband index back.